<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>flex</title>
    <style>
        div
        {
            width: 500px;
            height: 400px;
            border: solid 2px #000000;
            display: flex;
            margin: 0 auto;

        }

     div   section
        {
            width: 20px;
            height: 20px;
            background: red;
            margin: 20px ;
            float: left;

            justify-content: space-between;
        }
        section:nth-child(1)
        {
            flex: 1;
        }
        section:nth-child(2)
        {
            flex: 1;
        }
        section:nth-child(3)
        {
            flex: 1;
        }
    </style>
</head>
<body>
<div>
    <section></section>
    <section></section>
    <section></section>
</div>


</body>
</html>

    <style>
        div
        {
            width: 100%;
            height: 100px;
            border: solid 2px #000000;
            display: flex;
            margin: 0 auto;

        }

        div   button
        {
            /*width: 40%;*/
            /*height: 20px;*/
            background: red;
            /*margin: 1px ;*/
            float: left;

            justify-content: space-between;
            font-size: 30px;
        }
        button:nth-child(1)
        {
            background: red;
            flex: 1;
        }
        button:nth-child(2)
        {
            background: green;
            flex: 1;
        }
        button:nth-child(3)
        {
            background: gray;
            flex: 1;
        }
    </style>
</head>
<body>
<div>
    <button type="submit" value="提交">提交</button>
    <button></button>
    <button></button>
</div>


</body>
</html>